A. Field of the Invention
The present invention relates generally to optical modulators, and more particularly to surface normal mechanical optical modulators and methods for fabricating the same.
B. Description of the Prior Art
It is desirable in optical wavelength-division-multiplexing networks to have inexpensive light modulators that have high contrast and wide optical bandwidths. In certain cases, such as audio and video transmission, these modulators need only operate at frequencies up to several megahertz.
A modulation device particularly well suited for the above application is a surface normal micromechanical modulator. This device may be described as having a variable air gap defined by two layers of material. Typically, surface normal light modulators operate by changing the amount of light reflected in the surface normal direction, i.e., the direction normal to the substrate surface. This may be achieved by varying the variable air gap, which alters the optical properties of the device.
Compared with other modulation means, such as a laser, micromechanical modulators are limited in terms of modulation frequency. However, the micromechanical modulators are less expensive to implement and are readily fabricated on silicon substrates facilitating integration with silicon based electronics. Further, unlike the typical semiconductor laser, micromechanical modulators operate in a surface normal manner. This is an attractive feature since a device which operates in this manner requires less wafer space than a device, such as a typical semiconductor laser, in which the operating cavity is formed in the plane of the wafer. Many thousands of surface normal modulators may be formed on a single wafer, minimizing cost. Thus, where the operating frequency is limited, the micromechanical modulator may be the modulation device of choice.
One such micromechanical modulator has been described by Aratani et al. in xe2x80x9cProcess and Design Considerations for Surface Micromachined Beams for a Tuneable Interferometer Array in Silicon,xe2x80x9d Proceedings of the IEEE, Microelectromech Workshop, Ft. Laud., Fla., Feb. 7-10, 1993 at pages 230-35. This article, and all other articles referenced in this specification are herein incorporated by reference in their entirety. Aratani""s modulator is described as having a diaphragm mirror consisting of a polysilicon/silicon nitride multilayer supported by thin beams over a substrate, also partially mirrored by a polysilicon/silicon oxide multilayer. As a voltage is applied between the membrane and the substrate, the membrane is pulled toward the substrate. The device is said to behave as a Fabry-Perot interferometer wherein, given two mirrors having equal reflectivity, the reflectivity of the device approaches zero at the resonant wavelength of the cavity. As the membrane moves, altering the cavity, the reflectivity of the device rises. The change in reflectivity modulates the optical signal. While a large change in reflectivity is said to be achieved, the optical bandwidth of the optical resonator based modulator is limited. The contrast ratio of such a device falls off sharply as the wavelength of the incident light varies from the resonant wavelength of the device.
A second micromechanical modulator was described by Solgaard et al. in xe2x80x9cDeformable Grating Optical Modulator,xe2x80x9d Optics Letters, vol. 17, no. 9, pages 688-90 (1992). This modulator was described as having a reflection phase grating of silicon nitride beams which is coated with metal and suspended over a substrate which is also coated with metal. An air gap separates the grating and substrate. In the absence of a biasing voltage, the path length difference between the light reflected from the grating beams and that reflected from the substrate is equal to the wavelength of the incoming light. These reflections are therefore in phase, and the device reflects the light in the manner of a flat mirror. When a voltage is applied between the beams and the substrate, the beams are brought in contact with the substrate. The total path length difference between the light reflected from the grating beams and that reflected from the substrate changes to one half of the wavelength of the incident light. In this case, the reflections interfere destructively, causing the light to be diffracted.
The deformable grating optical modulator does not achieve a low reflectivity state. Rather, it switches to a diffracting state. In the diffracting state, incident light is scattered into higher-order diffraction modes of the grating, so that the amount of light reflected into the zero order (surface-normal) mode is minimized. Such diffraction may be an undesirable aspect of the deformable grating optical modulator. If the numerical aperture of the incoming fiber or detection system is large enough to pick up the higher order diffraction modes, a degradation in contrast will result. Further, if this device is implemented in a system using arrays of optical beams or fibers, a significant amount of optical crosstalk may be introduced.
A plot of reflectivity versus wavelength of the modulated signal for a modulator according to the prior art is shown by FIG. 1. The modulator is centered at approximately 1425 nanometers (nm) for varying thickness of the gap, i.e., from 10500 to 7400 Angstroms (xc3x85). Accordingly, the modulator of the prior art does not provide high contrast modulation for optical signals over a wide range of wavelengths, in particular from 1300 to 1600 nm.
Accordingly, there is a need for an apparatus for modulating an optical signal which provides high contrast modulation for optical signals over a wide range of wavelengths, in particular from 1300 to 1600 nm, and which does not introduce a significant amount of optical crosstalk, and methods for fabricating the same.
A method and apparatus for modulating an optical signal are disclosed. The apparatus, which may be formed on a semiconductor wafer or chip, comprises a membrane and a substrate, spaced to form an air gap. The membrane consists of two layers, and is suspended over the substrate by support arms. Bias is applied to the membrane and the substrate to create an electrostatic force to move the membrane towards the substrate. The layers of the membrane are characterized in that there is a relationship between the refractive indices of the layers and the refractive index of the substrate.
According to the present invention, the air gap, in the unbiased state, is approximately a multiple of one-quarter of a wavelength of the optical signal. Where the air gap is approximately an odd multiple of one-quarter wavelength, the membrane and air gap function as a high reflectivity coating. Where the air gap is approximately an even multiple of one-quarter wavelength, the membrane and air gap function as an anti-reflection coating. Under the action of bias, the membrane moves through one-quarter of a wavelength to an anti-reflection state or a maximum reflection state depending upon the state of the unbiased membrane. In the embodiments disclosed herein, the membrane does not contact the substrate.